Sensitizer containing gel-forming colloidal clay



Patented Apr. 27, 1943 umrso STATES PATENT OFFICE SENSITIZER CONTAININGGEL-FORMING COLLOIDAL CLAY 9 Claims.

This invention relates to improvements in the sensitized layers ofphotosensitive papers, and to means oi accomplishing these improvements.An object of this invention is the formation of sensitized layers withimproved properties, particularly sensitized layers which have higherprinting speed, which can be washed more rapidly, and which will giveimages at greater shapness, better definition, and improved clarity.

Among the types of sensitizers to which our invention can be applied areso-called "blueprint sensitizers, either of the negative or directpositive type; diazo" sensitizers: and direct line iron sensitizers,whch are cited merely as examples of specific types of sensitizers withwhich our invention can be used. In general, our invention can be usedwith any type of sensitized layer which is applied to a base of paper,cloth, or the equivalent from a solution in water, whatever the activelight sensitive chemicals may be.

In the ordinary process oi preparing blueprint papers, paper or othersuitable base is coated with a solution which comprises light-sensitiveferric compounds such as ammonium ierrioxslate, a ferricyanide salt, andusually so-called solubilizing" or buflering agents such as potassiumoxalate, which are used primarily to improve the keeping and washingquality of the sensitized paper. The customary practice is to pass thepaper stock over coating rollers, which apply the sensitizing, solution,after which the paper is dried. It is common practice to apply two coatsof the sensitizing solution before drying. The papers are subsequentlyexposed to light under a design, washed to remove unreacted chemicals,developed in a solution of an oxidizing agent such as potassiumdlchromate, and rinsed before drying. The resulting prints are negativesof the original design, with white lines on a blue background.

This process suflers from a number of disadvantages. In the flrst place,the sensitizer penetrates into the paper below the surface for aconsiderable distance. This has two undesirable effects: First, it slowsdown the action oi light on the sensitive chemicals appreciably, sincethese chemicals are partially concealed in the paper matrix; andsecondly-for the same reason, it makes washing slower than is desirableand often less thorough. In addition, partially because washing must beprotracted, some chemicals may wash or bleed irom the exposed to theunexposed areas. causing indistinct lines and images. The contrast isalso reduced thereby.

In other types oi sensitive papers of the gen- A. Crowley and m... B.Mullen, Chicago, asaignors to The Huey 00., Chicago,

No Drawing. Application October 22, 1941, Serial No. 416,046

eral type described above, the same difliculties occur also, in greateror less degree. The disadvantages generally tound in processes in whichan aqueous sensitizing solution is applied to pa- 5 per 01 similar base,and arising largely from penetration of the sensitizer into the paper,can be summarized briefly as follows:

(1) The speed is reduced by penetration; (2) the ease, speed, andcompleteness of washing are lowered; and (3) the possibility of bleedingand of indistinct lines is increased. All or some of these difllcultiesare found in blueprint papers, whether of the ordinary lemon-yellow typecontaining ierricyanide, or the blue-blue" type containingi'errocyanide; in diazo papers; in silver negative or brown printpapers; and in the various direct line iron papers.

Various means have been tried to eliminate these troubles By increasingthe concentration of the sensitizer, it is possible to increase itsviscosity and, therefore, to reduce penetration. However, this methodcauses great difllculty in the coating plant, since the highlyconcentrated solutions are subject to crystallization when slightevaporation or change in temperature occurs, and will, therefore, changein composition (affecting the nature of the sensitized layer), and mayeven deposit crystals on the paper web. The use of a hard-surfaced paperhas also been tried, which reduces penetration and actually makes 'itpossible to eliminate the loss in speed and slowness of washing causedby penetration. However, in practice it is not possible to use a paperwith a sufllciently hard-sized, non-absorbent surface to eliminate theseeffects entirely, since to do so would mean that the sensitized layer onthe paperwould be so thin that it would give grossly insufllcientpigmentation. Also, since with a paper of this type of surface there isno means of bonding the sensitizing chemicals to the paper,and they areactually laid on the surface, bleeding is actually increased rather thanreduced.

Some mean of attaching the sensitized layer firmly to the paper isrequired, and no better method for use with aqueous solution sensitizershas been found than penetration into the paper. The best that has beendone to date is to attempt to control the penetration by means of thesurface size on the paper.

A We have discovered a method whereby aqueous solution sensitizers ofthe types described above and other types of sensitizers in aqueoussolution can be applied to paper or similar base, which liminates inwhole or in part these objectionable features found hitherto. Ourinvention consists in the use of a sensitizing solution which containsbentonite or an equivalent type oi gelforining colloidal clay, which iscoated onto the paper or other base. By the use or such a sensitizer, itis possible to obtain a sensitized coating which is substantially on thesurface of the paper, so that itis in a position to react most rapidlywith the light; soluble unreacted chemicals can be washed out morereadily and completely, and as the bentonite (or its equivalent) servesto bind the chemicals to the paper, reaction products are less likely tobe washed into the unexposed areas.

In the practice of this invention, an aqueous sensitizing solution orsuspension containing the light-sensitive chemicals and bentonite isapplied to the surface of the paper, cloth, or other base by anysuitable means. One method of application is the use of coating rollerssimilar to those ordinarily used with aqueous sensitizers: by adjustingthe tension on the paper as it passes over the coating rollers, it ispossible to lay down a satisfaeio: y film, which is smoothed, and excessmaterial removed if necessary, by use of a squeegee rod or a doctor'sknife. Other methods of application can also be used, and will readilysuggest themselves to those skilled in the art; and our invention is notlimited to any method of application of the sensitized layer.

The amount of bentonite required varies with the nature of the paper orother base and with the nature of the sensitizing solution. The quantityof bentonite should be sutllcient to increase the viscosity of thesensitizer, and .to prevent substantially all penetration into thepaper, With relatively hard-surfaced papers, or papers with a relativelyimpervious size, this will hat-- urally require less bentonite than withsofter, more porous papers. Also, the nature and amounts of ions presentand also the relative order of mixing of the components of thesensitizer affect the viscosity and other character'- istics of thefinal sensitizer. In general, it has been found that from 1% to byweight of bentonite will give satisfactory effects.

In practicing this invention, it is preferable to use a relativelyhard-surfaced paper which has a low tendency to absorb water. Such apaper can be prepared by use of a glue size which is subsequently tannedby formaldehyde or other agents, by sizing with resins such asurea-iormaldehyde type, by use of other types of surface sizes, or bysupercalenderlng, or by combinations of these means or by other means.However, it is possible by proper control of the sensitiaer to obtaingood results even on papers of more open or porous structure, and ourinvention is notlimited to any particular type of paper.

We have mentioned the fact that bentonite increases the viscosity of thesensitlzer; however, this is not believed to be the sole explanation ofits effect in reducing penetration into the paper. If such were thecase, it would be possible simply by increasing the content 01'dissolved solids, for example by the addition of sugar, to increase theviscosity suiiiciently to reduce penetration. However, although this hasbeen attempted in the past, it is possible to achieve the eflect desiredonly to a limited extent by this method.

Bentonite and other gel-forming colloidal clays are hydrophlliccolloids, and when dispersed in water they reduce the amount oi free oravailable water. By so doing bentonite increases the viscosity of thesolution. but its eifects are not ascribable to the increased viscosityalone. but rather to the reduction in the free water, since all or alarge part of the water is linked to the bentonite (or to the ions) aswater by hydration. While this is believed to be the true explanation ofthe action of bentonite, our invention is not limited to thisexplanation.

Example I A. Bentonite g 10 Water ml 96 B. Ammonium oxalate ..g.. 6Citric acid g 6 Ferric ammonium oxalate g- 39 Potassium ferricyanide g..5 Water to make ml 244 (Density l3 B.)

Example I! A. Bentonite -g 16.00 Water ml 145.00 B. Oxalic acid(H2C2O42H2O) g 6.00

Potassium carbonate (KZCOJ-2H20) g 13.31 Potassium ierrocyanide g 15.00Potassium ferricyanide g 0.375 Ferric ammonium oxalate g 30.00

Water to make -ml 314 (Density 13.2 B.)

Example III A diazo sensitizer is prepared as follows:

A. Diazo sensitizer g 70.0 B. Bentonite g 3.0

Water A is added to B and mixed thoroughly. The mixture is coated on topaper, and the paper is exposed under a design to light. Development isefiectecl with an alkaline solution or a coupling agent consisting ofphloroglucinol, resoreinol, sodium carbonate, and water.

Diazo papers can also be prepared under this invention by incorporatingin the sensitize: the coupling compounds; the sensitizer must then beacid to prevent coupling. Such papers are developed after exposure bytreatment with a solution oi an alkali or by exposure to a moistatinosphere containing ammonia or other alkaline vapor.

. The term bentonite is intended to describe the colloidal clay roundalmost exclusively in Wyoming. The term is not a trade name, but ageneric name descriptive of the clay round in the Fort Benton area.There are other similar colloidal clays found in other parts or theUnited states and known as subbentonite clays and available for use.although with less eflectiveness in the use outlined in the foregoingdescription.

It will be understood that this invention is not restricted to theparticular sensitizers shown, nor to the types oi. sensitizers shown,which are cited merely as examples; and no unnecessary restrictions areto be assumed therefrom.

We claim:

1. A sensitizer comprising light-sensitive chemicals and a gel-formingcolloidal clay.

2. A sensitizer comprising an aqueous solution of light-sensitivechemicals and a gel-iorming colloidal clay.

3. A sensitizer comprising light-sensitive chemicals and gel-formingbentonite. A

4. A sensltizer comprislna an aq eous solution of light-sensitivechemicals'and gel-forming bentonite.

5. A sheet containing a layer 01' light-sensitive chemicals andgel-forming bentonite.

B. A sheet having a composite layer includin a sensitize'r andgel-forming bentonite, said bentonite acting to inhibit penetration ofthe chemicals into the structure.

7. A sheet having a composite layer including a sensitizer andgel-forming bentonite in the proportion or 1 to 10% by weight of thesensitizcr. said bentonite'acting to inhibit penetration of thesensitizer into the body of the sheet.

8. A sensitizer tor blueprint paper including a light-sensitive i'erriccompound and a minor proportion of a geldorming colloidal clay.

9. Blueprint paper comprising a sheet having a composite layer includinga sensitizer comprisin: light-sensitive ferric compound and a minorproportion 01 a gel-forming colloidal clay, said clay acting to inhibitpenetration or the sensitizer chemicals into the body oi the sheet.

CLYDE A. CROWLEY. JOHN B. MULLEN.

